Microwave amplitude modulator



MODULATED OUTPUT VSL H. JACOBS ETAL Filed Dec. 30, 1960 MODULATED LIGHTSOURCE MAGNITUDE MICROWAVE AMPLITUDE MODULATOR L IN M LLIMETERS INPUTJuly 2, 1963 FIG UNMODULATE FIG. 2

3 4 2 a m /& 0a 3 a 2 mvslvroRs,

HAROLD JACOBS FRANK A. BRAND JAMES D. MEI/VOL 5 MICHAEL A. BENANTL Z4?$405M? ATTORNEY,

l 2 L IN MILLIMETERS United States Patent 3,096,494 MICROWAVE AMPLITUDEMODULATOR Harold Jacobs, West Long Branch, and Frank A. Brand, Elberon,NJ., James D. Meindl, East Pittsburgh, Pa.,

and Michael A. Benauti, Mamaroneck, N.Y., assiguors to the United Statesof America as represented by the Secretary of the Army Filed Dec. 30,1960, Ser. No. 79,936 1 Claim. (Cl. 333-81) (Granted under Title 35, US.Code (1952), see. 266) Y he invention described herein may bemanufactured and used by or tor the Government tor governmental purposeswithout the payment of anyroyalty thereon.

This invention relates to microwave amplitude modulator-s, and inparticular to such apparatus utilizing semiconducting materialsinterposed in the path of the wave energy.

Hitherto, various arrangements have been suggested for operation ofmicrowave devices as amplitude modulators with little or no change inphase. However, such arrangements have generally involved verycomplicated circuitry, are very costly, cumbersome, require high powercapabilities and so are of limited applicability.

Accordingly, it is an object of this invention to provide improvedmicrowave amplitude modulators with little or no phase As far as isknown, there is no commercial product available which can satisfy therequirements of microwave amplitude modulators capable of maintainingvery narrow band transmission, in order to communicate the maximumintelligence over a given portion of the electromagnetic spectrum, in adesign which is as simple and compact as might be desired.

Accordingly, it is a further object of this invention to combine awaveguide and a semiconductor body into a simple compact unit which issusceptible of easy manufacture and which can be designed to fulfill allthe requirements mentioned above.

For a more detailed description of the invention, together with otherand further objects thereof reference is bad to the followingdescription taken in connection with the accompanying drawing in which:

FIG. 1 is a perspective view of a rectangular hollowpipe waveguideembodying the prinicples of the invention; and i FIGS. 2 and 3 aregraphs illustrating the results of numerous experiments which serve asthe basis of the present invention.

Before discussing specific embodiments of the invention, it will behelpful to develop some general principles.

It has been known for some time that when a semiconductor slab isinserted in a'rnicrowave field that a certain amount of absorptionoccurs. By varying the conductivity of the semiconductor, variations inabsorption and hence transmission of power is attained. However, in allof these cases, the geometrical dimensions of the semiconductor slab hasbeen ignored. The present invention is based on numerous theoretical andexperimental considerations which show that, in order to obtainmicrowavemodulation with little or no phase shaft by conductivitychanges in the semiconductor slab, the thickness dimension is critical.In other terms, it has been found when a semiconductor body is locatedacross a waveguide with its specified thickness dimension being in thesame direction as the propagated energy, that if the conductivity ismodulated amplitude modulation is attained with little or no phaseshift.

Referring to FIG. 1, there is shown a waveguide which may be of anydesirable configuration, as for example a waveguide having a rectangularcross section.

Electromagnetic waves are propagated through the "ice waveguide 10* withlinear polarization, its electric vector E as shown. This mode ofwaveguide propagation is denoted as TE mode, and this embodiment of theinvention will be explained on such an assumption, although theinvention is not altogether limited to this particular mode oftransmission. The TE mode may be launched in the waveguide by any ofseveral coupling arrangements well known in the The end 12 may also becoupled to a conventional receiver 13, in order to receive wavespropagated through the waveguide.

A semiconductor body 16 is located within the waveguide 10 and disposedat some region between the input end 14 and the output end 12 of thewaveguide, through which the electromagnetic wave is propagated.Semiconductor body 16 may be located midway between the H-plane ornarrow walls of waveguide 10 and spaced therefrom, or may extend acrossthe entire cross-section area of the waveguide. The thickness dimensionL of semiconductor body 16, the critical dimension according to thisinvention, extends along a portion of the waveguide length and isparallel to the path of wave propagated energy.

The conductivity of semiconductor 16 is modulated in a conventionalmanner by variable light or by variable junction injection of excessminority carriers from a modulation source. Since the conductivity ofsemiconductor body 16 is proportional to the intensity of the injectedcarriers, modulation of the light or the junction will cause amodulation of the conductivity, and therefore an amplitude modulation ofmicrowave energy traversing the waveguide 10. The carriers may beinjected by incident light through apertures in the H-plane or narrowwalls in the waveguide, or in the case of junction injection byhorizontal wires contacting the semiconductor body 16 and going outthrough holes in the narrow walls in waveguide 10. Modulation by lightis illustrated in FIG. 1. In the narrow wall 18, a small apelt-ure 20 isprovided as shown, which permits light directed from an intensitymodulated light source .22 to impinge upon the semiconductor body 16.The light from source 22 may be varied in any known manner.

in the case of modulation by light, absolute single crystals are notessential. Polycrystalline photosensitive semiconductor material, suchas cadmium sulphide or lead sulphide can be utilized as long as thesemiconductor materialh-as as long a liietime as possible. Experimentshave been made using semiconductor material having a lifetime range from1 microsecond up to 2000 microseconds.

Examples of semiconductor materials suitable for junction injection ofthe carriers are germanium, silicon and alloys or compounds made upfirom the elements in the III and V group in the periodic table ofelements. For purposes of the present example, a germanium body isselected, the intrinsic region preferably having a resistivity of atleast 5 ohm-centimeters or higher. The germanium surface is processedfor minimum surface recombination and the lifetime can be enhanced bythe presence of irm purity levels, as by copper trapping.

The present invention is based on numerous experiments which show thatthere is provided a microwave amplitude modulator having little or nophase shift comprising a hollcwpipe waveguide section through whichmicrowave energy can be propagated, a semiconductor body located withinthe waveguide in the path of said energy, the thickness of saidsemiconductor body being in the same direction as said propagatedenergy, said semiconductor body being characterized in that thethickness of said semiconductor body is calculated according to eitherone of the two following formulas:

and

where L=the thickness of the semiconductor body in millimeters, and Vk=the wavelength of the propagated energy through the semiconductor bodyin millimeters.

In order to better explain the operation of the microwave amplitudemodulator of this invention reference is made to the curves shown inFIGS. 2 and 3, wherein: 7 FIG. 2 is a representation of the magnitude ofthe ratio of electric field intensity, E transmitted through thegermanium slab 16 to the electric field incident, E upon the frontsurface as a function of thickness, L, and conductivity 0', and

FIG. 3 shows the phase angle, 6, of the electric field intensity, Etransmitted through the germanium slab, with respect to the electricfield incident on the front surface, E as a function of thickness, L,and conductivity, 0'.

To show the changes of conductivity and thickness on the changes intransmission of the electric field through the germanium body 16,calculations were made in accordance with the following parameters:

(a) E /E the magnitude of the ratio of the trans mitted electricintensity to the electric intensity in air incident upon the surface;

. (b) 0, the phase angle in radians of E with respect to in;

(0) a, the conductivity at 2, 3, 4, 6 and reciprocal ohm-meters; and

(d) L, the thickness of the germanium slab 16 in millimeters.

The specific values shown in FIGS. 2 and 3 are for the propagationcharacteristics of electromagnetic waves being transmitted through thegermanium body 16 at 10,000 megacycles per second. Using these data thefollowing information is shown:

In examining FIG. 2, if it is assumed that the germanium slab 16- has aconstant thickness L, it is seen that E /E will vary with conductivity.For instance, at 4 millimeters thickness, varying the'conductivity ofslab 16 from 0 :2 to 0': 10, by some physical means such as light oruniform injection of excess minority carriers, will decrease E /E fromabout 56 percent to 10 percent.

In FIG. 3 is shown the phase shift due to conductivity modulation of theelectric intensity. It is noted, that at 'a critical thickness 2.25millimeters, a first node exists due to the internal reflections, and asecond node of little phase shift starts at about 3.75 millimeters andextends to about 4.6 millimeters. In other words, if the conductivity ismodulated with either of these specific thicknesses, amplitudemodulation is attained with little or no change in phase.

Graphs were prepared similar to FIG. 2, formulated from calculationsderived from the propagation characteristics of waves transmittedthrough the germanium body 16 at various frequencies. In all instances,it was observed if the thickness in millimeters of the germanium body 16at the first and second nodes were divided by the wavelength inmillimeters in the germanium body, the result was two respectiveconstants .3 and .5. 1 This is exemplified by the following data:

. The invention is not restricted to the particular example describedand illustrated. It is to be understood that the same constants .3 and.5, respectively, have been found toexist, in free space or in aWaveguidc,'for silicon and semiconductor alloys or compounds withresistivity greater than 5 ohm-centimeters and suflicient lifetime dueeither to intrinsic action or to traps. It is also interesting to notethat these nodes in phase shift have been determined analytically andexperimentally verified when the semiconductor slab is operated in thereflection mode. The constants, however, are different than those foundfor the transmission mode.

While there has been described what is at present considered a preferredembodiment of this invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is therefore aimed in the appendedclaims to cover all such changes and modifications :as fall within thetrue spirit and scope of the invention.

What is claimed is:

A microwave amplitude modulator having little or no phase shift of thereflected waves comprising a hollowpipe waveguide of rectangularcross-section and operating in the TE mode through which microwaveenergy can be propagated, 1a germanium body having a resistivity of atleast 5 ohm-centimeters located within the waveguide in the path of saidenergy, the ratio of the dimension of said germanium body extending'inthe direction of said propagated energy to the wavelength of thepropagated energy through said germanium body being a con stant equal to.3, and said wavelength and said dimension being measured in the samedimensional units.

References Cited in the file of this patent UNITED STATES PATENTS2,974,223 Langberg Mar. 7., 1961 2,977,551 Gibson et al. Mar. 28, 1961

1. A MICROWAVE AMPLITUDE MODULATOR HAVNG LITTTLE OF NO PHASE SHIFT OFTHE REFLECTED WAVES COMPRISING A HOLLOW PIPE WAVEGUIDE OF RECTANGULARCROSS-SECTION AND OPERATING IN THE TE0,1 MODE THROUGH WHICH MICROWAVEENERGY CAN BE PROPAGATED, A GERMANIUM BODY HAVING A RESISTIVITY